1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a ROM memory which has a novel structure and from which data can hardly be copied.
2. Description of the Related Art
The memory cell array of a semiconductor memory, such as a ROM (Read Only Memory), comprises memory cells which are MOSFETs arranged in rows an columns. Each memory cell has its gate connected to a word line which extends in a row direction and its source and drain connected to a bit line which extends in a column direction. To read data from any desired memory cell, the bit line connected to the source and drain of the memory cell is selected, and the word line connected to the gate of the memory cell is set at high potential.
How to read data from a conventional ROM memory will be explained. The memory cell array of the ROM memory comprises memory cells which are MOS transistors arranged in rows and columns. Each memory cell has its gate connected to a word line, which in turn is connected to a row decoder. Each memory cell has its drain connected to a bit line, which in turn is connected to a column decoder by a selecting transistor. The bit line is connected to the input terminal of a sense amplifier, the output of which is coupled to the input terminal of an output circuit. Upon receipt of an address, the column decoder selects one of the bit lines provided, in accordance with the address. The potential of the bit line thus selected is applied to the input terminal of the sense amplifier. Upon receipt of an address, the row decoder selects one of the word lines provided, in accordance with the address. The normal power-supply voltage is applied to the gate of the memory cell which is connected to both the selected bit line and the selected word line. As a result, data is read from this memory cell.
The conventional semiconductor device of the type has the problem that data stored in it can easily be copied.